1. Field of the Invention
The present invention relates to a method for the continuous production and coating of self-adhesive compositions based on SBC with at least one pharmaceutically active substance.
2. Discussion of Background Information
Fundamental to the profile of performance requirements of pressure sensitive adhesive systems and the pressure sensitive adhesive articles produced with them (such as patches, for example) are the two physical phenomena of adhesion and cohesion of the pressure sensitive adhesive layers. Adhesion is dealt with in the technical jargon using the terms instant bond strength (tack) and bond strength (peel strength) and describes by definition the terms “self-adhesive”, “pressure sensitive adhesive” and/or “pressure sensitive adhesive tapes”, i.e., permanent adhesive bonding under “gentle applied pressure”.
Especially in the case of pressure sensitive adhesives based on rubber, this property is obtained by mixing in tackifying resins (tackifiers) and plasticizers having relatively low molecular weights.
The second defining property of the pressure sensitive adhesives is their simple residue-free redetachability after use. This property is determined essentially by the high molecular mass rubber fractions as the elastomer component, which give the system, in the form of cohesion (internal strength), the required strength under shear stress, which is of particular significance for the use of the products under mechanical load.
The performance of the pressure sensitive adhesive is, therefore, critically determined by the balanced proportion of adhesion properties and cohesion properties and by compatibility, homogeneity, and stability of the blend of components with extremely high and relatively low average molecular weights, something which is relatively easy to achieve when the composition is produced in industry-standard mixers and kneading machines using solvents.
The advantage of foregoing the use of solvents lies essentially in the simplification of the coating method. The avoidance of flammable solvents does away with the need for the drier units, with their high energy consumption for the evaporation and recovery of the solvents, and with the need to use explosion-protected units. Hot-melt coating units are compact and permit much higher coating speeds. The technology is an environment-friendly one in which there are no solvent emissions. Furthermore, no unwanted solvent residues remain in the self-adhesive composition. This is the reason for the reduction in the allergenic potential of the product.
For solvent-free compounding, the prior art makes use predominantly of block copolymers having polystyrene block fractions, or natural and/or synthetic rubbers.
Owing to the high molecular mass fractions of the rubber (with MW≧3*105 g/mol), solvent-free natural rubber self-adhesive compositions cannot be processed by the hot-melt pressure sensitive adhesive technology, or else the rubbers used must be reduced in their molecular weight (broken down) severely before processing.
The deliberate industrial process of rubber breakdown under the combined action of shear stress, temperature, and atmospheric oxygen is referred to in the technical literature as mastication and is generally carried out in the presence of chemical auxiliaries, which are known from the technical literature as masticating agents or peptizers, or, more rarely, as “chemical plasticizing agents”.
In rubber technology, the mastication step is necessary in order to make it easier to integrate the additives.
Mastication is a term used in rubber technology for the breaking down of long-chain rubber molecules in order to increase the plasticity and/or reduce the (Mooney) viscosity of rubbers. Mastication is carried out by treating, in particular, natural rubber in kneading apparatus or between rolls at very low temperatures in the presence of mastication aids (masticating auxiliaries). The high mechanical forces which act lead to a “tearing apart” of the rubber molecules, with the formation of macroradicals, whose recombination is prevented by reaction with atmospheric oxygen. Mastication aids such as aromatic or heterocyclic mercaptans and their zinc salts or disulfides accelerate the mastication process by promoting the formation of primary radicals. Activators such as metal (iron, copper, cobalt) salts of tetraazaporphyrins or phthalocyanines enable the mastication temperature to be lowered. For the mastication of natural rubber, mastication aids are used in amounts of from about 0.1 to 0.5% by weight in the form of masterbatches, which facilitate a uniform distribution of this small amount of chemicals within the rubber composition.
Mastication must be clearly distinguished from the breakdown known as degradation which results in all of the standard solvent-free polymer technologies, such as compounding, conveying, and coating in the melt.
Degradation is a collective term for various processes which change the appearance and properties of plastics. Degradation may be caused, for example, by chemical, thermal, oxidative, mechanical or biological influences or else by the effect of rays (such as (UV) light). Examples of consequences are oxidation, chain cleavages, depolymerization, crosslinking, and/or elimination of side groups of the polymers. The stability of polymers with respect to degradation may be increased by using additives, for example, by adding stabilizers such as antioxidants or light stabilizers.
A variety of routes to the solvent-free production and processing of rubber pressure sensitive adhesives have been described.
The patent CA 698 518 describes a process for producing a composition by adding high proportions of plasticizer and/or by simultaneously strong mastication of the rubber. Although this method may be used to obtain pressure sensitive adhesives having an extremely high tack, a user-compatible shear strength can be achieved only to a limited extent, even with a relatively high level of subsequent crosslinking, owing to the relatively high plasticizer content or else to the severe breakdown in molecular structure of the elastomer to a molecular weight average of MW≦1 million.
The use of polymer blends, where, besides nonthermoplastic natural rubber, use is also made of block copolymers, in a ratio of approximately 1:1, is essentially an unsatisfactory compromise solution, since it results neither in high shear strengths when the self-adhesive tapes are used at relatively high temperatures nor in significant improvements relative to the properties described in the patent.
The use of exclusively non-thermoplastic rubbers as the elastomer component in the formulation of pressure sensitive adhesives with the existing cost advantage possessed by, for example, natural rubbers over the standard commercial block copolymers, and the outstanding properties, especially the shear strength of natural rubber and of corresponding synthetic rubbers, is also set out at length in the patents WO 94 11 175 A1, WO 95 25 774 A1, and WO 97 07 963 A1 and, correspondingly, U.S. Pat. Nos. 5,539,033 A and 5,550,175 A.
In these cases, the additives customary in pressure sensitive adhesive technology, such as tackifier resins, plasticizers, and fillers, are described.
The production method disclosed in each case is based on a twin screw extruder which permits compounding to a homogeneous pressure sensitive adhesive blend with the chosen process regime, involving mastication of the rubber and subsequent gradual addition of the individual additives with an appropriate temperature regime.
The mastication step of the rubber, which precedes the actual production process, is described at length. It is necessary and characteristic of the process chosen, since with the technology chosen therein it is indispensable to the subsequent integration of the other components and to the extrudability of the blended composition. Also described is the feeding in of atmospheric oxygen, as recommended by R. Brzoskowski, J. L. and B. Kalvani in Kunststoffe 80 (8), (1990), p. 922 ff., in order to accelerate mastication of the rubber.
This procedure makes it absolutely necessary to practice the subsequent step of curing by electron beam crosslinking (EBC) and to use reactive substances as EBC promoters in order to achieve an effective crosslinking yield.
Both method steps are described in the abovementioned documents, but the EBC promoters chosen also tend toward unwanted chemical crosslinking reactions at elevated temperatures. This limits the use of certain tackifying resins and the use of the self-adhesive compositions produced, in particular for pharmaceutical applications.
It is an object of the present invention to provide a method with which self-adhesive compositions based on SBC and comprising at least one active pharmaceutical substance can be produced continuously without solvent and, if desired, can be coated in-line without the need for property-impairing mastication of the SBC.